This invention concerns an apparatus for manufacturing plastic bags from at least one double-ply thermoplastic sheeting both of the plies of which have closure strips formed on the facing inner edges so as to form a snap-fastener-like closure of the bag openings. The apparatus has a conveyor device which feeds the sheeting to a welding station in which a welding ledge, extending transverse to the direction of conveyance, separates the individual bags from the sheeting by separation welding.
The starting material for the manufacture of such bags is an extruded tube, on the inside of which the closure strips are producing during extrusion. The short side between the closure strips is cut open before the production of the bags so that an edge strip which forms a gripping piece is produced between each closure strip and the cut edge. The tube which has been cut open in this manner is generally referred to as a half-tube.
During manufacture of the bags the half-tube is pressed together flat such that the closure strips inter-engage, and the resulting closed half-tube is introduced into the welding station, where it is divided by separation welding into the individual bags by the welding ledge extending transverse to the direction of conveyance and thus transverse to the longitudinal axis of the half-tube.
When forming bags without closure strips, the separation welding process time is extremely short so that, with thin material, 250 welds per minute are possible. However, when forming bags with closure strips, the accumulation of material in the region of the closure strips must be cut through in the separation welding process. Much more time is required to melt through the thickness of the closure strips so that a substantially smaller number of bags can be welded per minute.
In order to increase the capacity of the aforementioned apparatus for the manufacture of plastic bags with closure strips it is known to deform the closure strips using heated press dies or ultrasonics at a station located upstream of the welding station so that the thickness of the closure strips at the point of welding is reduced to a value which can more easily be separated by welding. Other known methods include shortening the separating process in the region of the welding ledge by changing the cutting angle of the separating welding ledge.
All of these measures, however, are unsatisfactory and do not succeed in substantially improving the output per minute of the apparatus.